Producing bis(alicyclic) thioethers

ABSTRACT

3,3&#39;-Thiobis(2,5-dihydrothiophene-1,1-dioxides) on heating with a reactive Diels-Alder dienophile undergo a reaction producing thioethers having two six-membered alicyclic rings in the molecule, such as 4,4&#39;-thiobis(3,6-dihydrophthalic acids) or the esters, anhydrides or imides thereof, and 4,4&#39;-thiobis(1,2,3,6-tetrahydrophthalic acids) or the esters, anhydrides or imides thereof. Both rings of the dihydrothiophene dioxides participate in the reaction, apparently by forming a tetraene structure which co-reacts with two equivalents of the dienophile to form an adduct in which each six-membered ring has one or two olefinic double bonds. During the course of the reaction, the thioether bridge remains intact. Some of the bis(alicyclic) thioethers are readily converted to the corresponding bis(aromatic)thioethers by use of known aromatization methods and systems. Various utilities for the products of the foregoing reactions are described.

This application is a division of application Ser. No. 917,773, filedOct. 10, 1986, now U.S. Pat. No. 4,798,900.

BACKGROUND

The only published route to 4,4'-thiobis(phthalic acid) derivativesinvolves treatment of 4-substituted N-alkylphthalimides with sodiumsulfide followed by hydrolysis and dehydration. See Evans et al, PolymerPreprints (American Chemical Society, Div. Polymer Chem.) 1984, 25, 268.Preparation of the starting materials for this route most readilyrequires electrophilic substitution reactions of phthalic anhydridefollowed by separation of 3- and 4-substituted isomers and imideformation. See Williams et al, J. Org. Chem. 1977, 42, 3414.

THE INVENTION

This invention involves the discovery, inter alia, that3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxides) on heating with areactive Diels-Alder dienophile undergo a reaction producing thioethershaving two six-membered alicyclic rings in the molecule, such as4,4'-thiobis(3,6-dihydrophthalic acids) or the esters, anhydrides orimides thereof, and 4,4'-thiobis(1,2,3,6-tetrahydrophthalic acids) orthe esters, anhydrides or imides thereof. Both rings of thedihydrothiophene dioxides participate in the reaction, apparently byforming a tetraene structure which co-reacts with two equivalents of thedienophile to form an adduct in which each six-membered ring has one ortwo olefinic double bonds. During the course of the reaction, thethioether bridge remains intact.

The utility of similar 2,5-dihydrothiophene-1,1-dioxides--which ofcourse-do not contain a thioether bridge--as masked dienes inDiels-Alder reactions is well established. See

Tsuji et al., Bull. Chem. Jpn., 1985, 58, 1603;

Chou et al, J. Chem. Soc., Chem, Comm., 1985, 236;

Charlton et al, Can. J. Chem.. 1973, 51, 1852;

Fieser and Fieser, Reagents for Organic Synthesis, Wiley IntersciencePub., N.Y., 1969, 2, 668;

Gundermann et al, Angew. Chem. Internat. Ed., 1966, 5, 668;

Cope et al., J. Am. Chem. Soc., 1961, 83, 3859; and

Cava et al., J. Am. Chem. Soc., 1959, 81, 4266.

The bis(alicyclic) thioethers produced by the above process can be usedfor a number of applications. For example, they can be used aspesticides, herbicides and plant growth regulants. In addition they maybe employed as extreme pressure additives and corrosion inhibitors inlubricating oils, especially when used in the form of imides or esters.Some of the bis(alicyclic) thioethers are readily converted to thecorresponding bis(aromatic)thioethers by use of known aromatizationmethods and systems such as are described or referred to in March,Advanced Organic Chemistry, John Wiley & Sons, New York, 1985, pages1052-4 (and references cited therein), and Fatma, et al., J. Chem. Res.(M), 1984, 2658. (It will be understood of course that the presence ofthe divalent sulfur in the thioethers precludes use or precious metalcatalysts for effecting aromatization.) Thiobis(phthalimides) andthiobis(phthalic anhydrides), which can be formed in this manner, havebeen reported to be useful as antioxidants in rubber, hydrocarbon oils,polypropylene, etc., as curing agents for epoxy resins, as intermediatesfor polyester resins or polythioetherimides, as plasticizers, and asfire retardants--see U.S. Pat. No. 3,989,712; Japan Kokai Tokkyo KohoNo. 60,188,368 (Sep. 25, 1985) [Chemical Abstracts 104:30428x]; andEvans et al, Polymer Preprints (American Chemical Society, Div. PolymerChem.), 1984, 25(1) 268-9. In some instances use of conventionalaromatization techniques results not in aromatization of thesix-membered rings, but in formation of six-membered diene ring systems.These products may be used as antioxidants, as corrosion inhibitors andas pesticides, herbicides and plant growth regulants. And thebis(alicyclic) thioethers formed by the process of this invention canreadily be converted to the corresponding sulfones, which may be used asrust inhibitors, pesticides, herbicides, and plant growth regulants.

The specific type of product formed in the above process is dependentupon the type of dienophile used in the reaction. For example, maleicanhydride and its hydrocarbon-substituted congeners (citraconicanhydride, dimethylmaleic anhydride, phenylmaleic anhydride, etc.) giverise to the formation of 4,4'-thiobis(1,2,3,6-tetrahydrophtalicanhydrides). Maleic acid and its hydrocarbon-substituted congeners(citraconic acid, etc.) form 4,4'-thiobis(1,2,3,6-tetrahydrophthalicacids), whereas the esters of maleic acid and itshydrocarbon-substituted congeners produce4,4'-thiobis(1,2,3,6-tetrahydrophthalic acid esters).

When the dienophile is maleimide, a hydrocarbon substituted congenerthereof (e.g., 2-methylmaleimide, 2,3-dimethylmaleimide, etc.) or anN-substituted maleimide in which the substituent is a carbon-bondedorgano group (e.g., N-methylmaleimide, N-phenylmaleimide,N-p-tolylmaleimide, N-cyclohexylmaleimide, N-ethyl-2-methylmaleimide,etc.), the product of the reaction is a4,4'-thiobis(l,2,3,6-tetrahydrophthalimide) in which each nitrogen atomis substituted by a hydrogen atom or by the N-organo substituent of theN-substituted maleimide.

Use of acetylene dicarboxylic acid or its esters as the dienophileresults in the production of 4,4'-thiobis(3,6-dihydrophthalic acid) orits esters.

If the dienophile is bromomaleic acid or bromomaleic anhydride or anester of bromomaleic acid, the product of the reaction is4,4'-thiobis(3,6-dihydrophthalic acid) or4,4'-thiobis(3,6-dihydrophthalic anhydride or an ester of4,4'-thiobis(3,6dihydrophthalic acid).

Any unsaturated compounds may be used that participate in Diels-Alderreactions, such as those containing the C═CX group, where x=CHO, COR,COOH, COOR, COCl, COAr, CN, NO₂, Ar, CH₂ Cl, CH₂ NH₂, CH₂ CN, CH₂ COOH,halogen, and the like as referred to in Holmes, Org. React., 4, 60-173(1948), the disclosure of which is incorporated herein by reference.

The 3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxides) used in the processmay be represented by the general formula ##STR1## wherein the rings maybe substituted by inert substituents such as alkyl, cycloalkyl, aryl,aralkyl, cycloalkylalkyl, and the like. The substituents should ofcourse be positioned and sized so that they do not prevent the desiredcyclization reaction by virtue of steric hindrance. Methods suitable forthe synthesis of such compounds are reported in the literature, e.g.,Lewis and Emmons, J. Org. Chem. 1966, 31, 3572, all disclosure of whichis incorrated herein by reference. Use of3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxide itself is preferredbecause of the ready availability and low cost of butadiene sulfone(3-sulfolene), the raw material from which3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxide is produced.

When heating 3,3'-thiobis(2,5,-dihydrothiophene-1,1-dioxide totemperatures within the range of 100° to 200° C. a thiotetraene, viz.,2,2'-thiobis(l,3-butadiene), is produced. The formation of thisthiotetraene was observed by GC/MS analysis. Attempts to isolate thethiotetraene resulted in polymerization even in the presence of apolymerization inhibitor.

To conduct the reaction between the3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxide and the dienophile, oneshould heat the system to a temperature sufficient to cause cyclizationto occur. By analogy with the thiotetraene formation, the temperature ispreferably maintained between about 100° and about 200° C. wherebypresumably the thiotetraene is formed as a transitory intermediate whichundergoes cyclization with the dienophile whereby two six-membered ringsare formed with the thiotetraene molecule. Most preferably temperaturesof at least about 140° C. and no higher than about 200° C. are used, asthis provides a suitably rapid rate of reaction.

It is convenient to conduct the reaction in an indifferent (innocuous;inert) liquid solvent such as a paraffinic, cycloparaffinic or aromatichydrocarbon, dimethylsulfoxide, dimethylformamide, dimethylacetamide,tetrahydrofuran, diglyme, dibutyl ether, a chlorinated hydrocarbon,acetonitrile, an organic alcohol, ethyl acetate, and the like.

The following examples are illustrative of the practice of variousembodiments of this invention. In the Examples: Melting points weredetermined with a Fisher-Johns hot stage or a Mel-Temp melting pointapparatus and are uncorrected. NMR spectra were recorded on a VarianEM-390 or a GE/NIC NT-360 spectrometer. Chemical shifts are reported inparts per million relative to tetramethylsilane. Infrared spectra wererecorded on a Perkin Elmer 983 spectrophotometer. Mass spectra wereobtained on a Finnigan 4023 gas chromatograph/mass spectrometer equippedwith a 50-m SE-52 fused silica capillary column. Elemental analyses wereperformed by Galbraith Laboratories, Knoxville, Tenn.

Examples 1 through 10 illustrate methods for the synthesis of thioetherbridged di- and tetrahydrophthalic acid derivatives, such as the esters,anhydrides, and imides thereof.

EXAMPLE 1 4,4'-Thiobis(1,2,3,6-Tetrahydrophthalic Anhydride)

A mixture of 10.0 g (37.5 mmol) of3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxide) (DTS) and 7.35 g (75.0mmol) of maleic anhydride in 25 mL of mixed xylenes (bp 139°-142° C.)was heated at reflux for 2 hours and allowed to cool to roomtemperature. After decantation of the xylenes the lower, oily layer wasdissolved in 25 mL of hot dichloromethane. The resulting brown solutionwas filtered through glass fiber paper, reduced in volume to 20 mL bydistillation, and cooled to yield 4.06g (32% yield) of a higher-meltingisomer of 4,4'-thiobis(l,2,3,6-tetrahydrophthalic anhydride): mp180°-182° C.; ¹ H NMR (CDCl₃) 2.40-2.74 (m, 8H), 3.47-3.64 (m, 4H),5.95-6.01 (m, 2H); ¹³ C NMR (DMSO-d₆) 24.9(t), 28.8(t), 39.1(d),40.3(d), 128.5(d), 130.9(s), 174.5(s), 175.0(s); IR (KBr) 3433 (H₂ O),2960, 1841, 1773, 1714, 1331, 1311, 1234, 1193, 1100, 1086, 1009, 967,938, 925 cm⁻¹. Anal. Calcd. for C₁₆ H₁₄ O₆ S: C, 57.48; H, 4.22. FoundC, 57.29; H, 4.17.

Addition of diethyl ether to the mother liquor and cooling afforded 2.16g (17%) of a lower-melting isomer of4,4'-thiobis(l,2,3,6-tetrahydrophthalic anhydride): mp 166°-169° C.; ¹ HNMR (CDC₃) 2.40-2.73 (m, 8H), 3.45-3.65 (m, 4H), 6.05-6.15 (m, 2H); ¹³ CNMR (DMSO-d₆) 25.0(t), 28.6(t), 40.0(d), 40.2(d), 129.2(d), 130.7(s),174.6(s), 175.0(s); IR (KBr) 3425 (H₂ O), 2942, 1845, 1774, 1716, 1242,1225, 119, 1102, 1091, 1035, 1009, 966, 941, 925, cm⁻¹. Anal Calcd. forC₁₆ H₁₄ O6S.1/2H₂ O: C, 55.97; H, 4.40. Found: C, 56.11; H, 4.42.

Concentration of the mother liquor gave 4.73g of brown glass.

EXAMPLE 2 4,4'-Thiobis(1,2,3,6-Tetrahydrophthalimide)

A mixture of 1.0 g (3.8 mmol) of DTS and 0.73 g (7.5 mmol) of maleimidein 3 mL of mixed xylenes (bp 139°-142° C.) was heated at reflux for 1hour and allowed to cool to room temperature. After decantation of thexylenes the remaining off-white solid was triturated with 15 mL ofboiling dichloromethane and filtered to give 1.2 g of white solid.Crystallization from methanol afforded 0.80 g (64% yield) of4,4'-thiobis(1,2,3,6-tetrahydrophthalimide): mp 204-°208° C.; ¹ H NMR(CDCl₃ /DMSO-d₆) 2.20-2.55 (m, 8H), 3.05-3.20 (m, 4H), 5.80-5.87 (m, 1H,isomer a), 5.95-6.00 (m, 1H, isomer b); ¹³ C NMR (Me₂ SO-d₆), pairedsignals due to isomer mixture, 24.7 and 24.9 (t), 28.7 and 28.9 (t),39.4 and 39.5 (d), 40.5 and 40.6 (d), 128.2 and 129.1 (d), 130.9 and131.0 (s), 180.5 and 180.6 (s), 181.1 and 181 2 (s); IR (KBr) 3431,3207, 2927, 1781, 1703, 1359, 1336, 1314, 1176, 787, 577 cm⁻¹. Anal.Calcd. for C₁₆ H₁₆ N₂ O₄ S.1/2H₂ O: C, 56.29; H, 5.02, N 8.21. Found: C,56.68; H, 5.18, N, 8.16.

EXAMPLE 3 4,4'-Thiobis(N-Phenyl-1,2,3,6-Tetrahydrophthalimide)

A mixture of 5.0 g (19 mmol) of3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxide) and 6.5 g (38 mmol) ofN-phenylmaleimide in 20 mL of mixed xylenes (bp 139°-142° C.) was heatedto reflux for 1 hour and allowed to cool to room temperature to give agel. This was warmed with stirring until a slurry resulted. Filtrationafforded a solid which was dissolved in 100 mL of hot dichloromethaneand filtered through a glass fiber paper. The filtrate was reduced involume to 50 mL by distillation, treated with diethyl ether, and cooledto give 3.0 g (33% yield) of4,4'-thiobis(N-phenyl-1,2,3,6-tetrahydrophthalimide): mp 212°-217° C.; ¹H NMR (CDCl₃) 2.42-2.80 (m, 8H), 3.19-3.32 (m, 2H), 5.95-6.02 and6.10-6.19 (m, 2H, 2 isomers), 7.20-7.50 (m, 10H); ¹³ C NMR (CDCl₃),paired signals due to isomer mixture, some aromatic signals overlap,25.3 and 25.6 (t), 29.1 and 29.7 (t), 38.6 and 38.8 (d), 39.6 and 39.8(d), 126.2 (d), 126.3 (d), 128.6 (d), 128.8 (s), 129.1 (d), 129.5 (s);IR (KBr) 3453 (H2O), 2918, 1709, 1597, 1494, 1381, 1199, 1180, 769, 694,586 cm⁻¹. Anal. Calcd. for C₂₈ H₂₄ N₂ O₄ S.1/2H₂ O: C, 6813, H, 5.11; N,5.68. Found: C, 68.24; H, 5.22; N, 5.69.

The mother liquor was stripped of solvent in vacuo to give 2.5 g ofbrown solid. NMR analysis indicated this contained mostly4,4'-thiobis(N-phenyl-1,2,3,6-tetrahydrophthalimide).

EXAMPLE 4 4,4'-Thiobis(3,6-Dihydrophthalic Anhydride)

A mixture of 1.0 g (3.8 mmol) of DTS, 1.7 g (7.5 mmol) of bromomaleicanhydride, and 6 mL of mixed xylenes (bp 139°-142° C.) was heated toreflux for 1 hour. Vigorous foaming occurred and acidic fumes(presumably HBr) were given off. The mixture was allowed to cool to roomtemperature, filtered to remove a little brown powder, and the filtratewas concentrated in vacuo to give an orange semisolid. Trituration ofthis residue with 10 mL of boiling dichloromethane followed byfiltration afforded 0.67 g (52% yield) of yellow, solid4,4'-thiobis(3,6-dihydrophthalic anhydride): mp 227°-230° C. (with gasevolution); IR (KBr) 3436, 2923, 1842, 1787, 11691, 1415, 1263, 1061,900, 714 cm⁻¹.

EXAMPLE 5 4,4'-Thiobis(3,6-Dihydrophthalic Anhydride)

A mixture of 1.0 g (3.8 mmol) of DTS, 1.7 g (7.5 mmol) of bromomaleicanhydride, 1.6 g (15 mmol) of sodium carbonate, and 6 mL of mixedxylenes (bp 139°-142° C.) was heated to reflux for 1 hour. Vigorousfoaming occurred. The mixture was filtered hot and the solids werewashed with 10 mL of toluene. The combined filtrates were concentratedin vacuo to give an orange solid. Trituration of this residue with 10 mLof boiling dichloromethane followed by filtration afforded 0.82g (65%yield) of 4,4'-thiobis(3,6-dihydrophthalic anhydride).

EXAMPLE 6 4,4'-Thiobis(3,6-Dihydrophthalic Anhydride)

A solution of 0.50 g (1.9 mmol) of DTS and 0.85 g (3.8 mmol) ofbromomaleic anhydride in 3 mL of dimethyl sulfoxide was heated at130°-150° C. for 1 hour and allowed to cool to Concentration in a streamof dry nitrogen afforded a residue which, by NMR analysis, contained4,4'(thiobis(3,6-dihydrophthalic anhydride).

EXAMPLE 7 4,4'-Thiobis(Dimethyl 3,6-Dihydrophthalate)

Nitrogen was bubbled through a mixture of 5.09 g (19 mmol) of DTS and5.4 g (38 mmol) of dimethyl acetylene dicarboxylate in 15 mL of mixedxylenes (bp 139°-142° C.) while it was heated to reflux by means of ahot oil bath for 1 hour. The mixture was cooled to room temperature andthe xylenes were removed by distillation at 25 torr. The resultingviscous orange oil was purified by flash chromatography on l50 g ofsilica gel eluted with 0-1% methanol in dichloromethane to give 4.6 g(58% yield) of pale yellow, waxy 4,4'-thiobis(dimethyl3,6-dihydrophthalate). An analytical sample was obtained bycrystallization from toluene: mp 101°-103° C.; ¹ H NMR (CDCl₃);3.04-3.20 (m, 8H), 3.81 (s, 12H), 5.95-6.08 (m, 2H); ¹³ C NMR (CDCl₃)29.5 (t), 31.5 (t), 52 1 (q. 2 carbons), 125.8 (s), 126.3 (d), 131.4(s), 131.8 (s), 167.2 (s), 267.8 (s); IR (KBr) 3426 (H₂ O), 2955, 1715,1668, 1440, 1413, 1272, 1255, 1140, 1064, 952, 748 cm⁻¹. Anal Calcd. forC₂₀ H₂₂ O₈ S: C, 56.86; H, 5.25. Found: C, 56.76; H, 5.27.

EXAMPLE 8

4,4'-Thiobis(1,2,3,6Tetrahydrophthalic Anhydride)

A mixture of 1.0 g (3.8 mmol) of DTS and 0.74 g (7.5 mmol) of maleicanhyride in 5 mL of toluene was heated at reflux for 2 hours and allowedto cool to room temperature. After decantation of the toluene the lower,oily layer was dissolved in 10 mL of dichloromethane. The resultingbrown solution was filtered, reduced in volume to 4 mL by distillation,and cooled to yield 0.65 g (53% yield) of4,4'-thiobis(l,2,3,6-tetrahydrophthalic anhydride.

EXAMPLE 9 4,4-Thiobis-(1,2,3,6-tetrahydrophthalimide)

A mixture of 25.0 g (93.9 mmol) of DTS, 18.2 g (188 mmol) of maleimide,and 75 mL of mixed xylenes (bp 139°-142° C.) was stirred vigorously andheated to reflux for 30 min. Decantation of the xylenes followed bytrituration of the solid residue with 50 mL of dichloromethane affordeda white solid which was recrystallized from tetrahydrofuran-methanol togive 24.6 g (79% yield) of 4,4'-thiobis-(1,2,3,6-tetrahydrophthalimide)as a 1:1 mixture of isomers.

EXAMPLE 10 4,4'-Thiobis-(N-phenyl-1,2,3,6-tetrahydrophthalimide)

A mixture of 1.0 g (3.8 mmol) of DTS, 1.3 g (7.5 mmol) orN-phenylmaleimide, and 4 mL of mixed xylenes (bp 139°-142° C.) washeated at reflux for 1 h and allowed to cool to room temperature. Afterdecantation of the xylenes the lower, oily layer was subjected to flashchromatography on 100 g of silica gel (eluted with 1% methanol indichloromethane) to give 1.4 g (75% yield) of4,4'-thiobis-(N-phenyl-1,2,3,6-tetrahydrophthalimide) as a 1:1 mixtureof isomers.

Example 11 illustrates the use of the thioether bridged di- andtetrahydrophthalic acid derivatives, such as the esters, anhydrides, andimides thereof as intermediates in the synthesis of the correspondingsulfones.

EXAMPLE 11 4,4'-Bis(N-Phenyl-1,2,3,6-Tetrahydrophthalimide) Sulfone

A slurry of 1.0 g (2.1 mmole) of4,4'-thiobis(N-phenyl-1,2,3,6-tetrahydrophthalimide) in 25 mL of glacialacetic acid and 5 mL of 30% hydrogen peroxide was heated at reflux for30 minutes. During this time the mixture became homogeneous, thenheterogeneous as a crystalline solid separated. The solid was removed byfiltration, washed with 10 mL of water, 10 mL of 10% sodium carbonate,20 mL of water, and 20 mL of absolute ethanol, and dried in vacuo togive 0.70 g (65% yield) of4,4'-bis(N-phenyl-(1,2,3,6-tetrahydrophthalimide) sulfone: mp 298°-299°C.; decomposition point (gas evolution) 330° C.; IR (KBr) 3451, 3080,2960, 1703, 1493, 1385, 1305, (SO₂), 1195 (SO₂) cm⁻¹. Anal Calcd. forC₂₈ H₂₄ N₂ O₆ S: C, 65.10; H, 4.68; N, 5.42. Found: C, 64.93; H, 4.80;N, 5.45.

Examples 12 through 16 illustrate the production of4,4'-thiobis(phthalic acid) derivatives by aromatization of thethioether bridged di- and tetrahydrophthalic acid derivatives, anddemonstrate the ease with which such aromatization can be effected usinga 4,4'-thiobis(tetrahydrophthalimide) having an organo substituent oneach nitrogen atom or an ester of 4,4'-thiobis(3,6-dihydrophthalicacid).

EXAMPLE 12 4,4'-Thiobis(N-Phenylphthalimide)

A mixture of 200 mg (0.41 mmol) of4,4'-thiobis(N-phenyl-1,2,3,6-tetrahydrophthalimide) and 53 mg (1.7mmol) of elemental sulfur was heated to 240°-260° C. (sand bath) for 20minutes and cooled to room temperature. The resulting brown residue waspurified by preparative TLC on silica gel (eluent: 1% methanol indichloromethane) to give 61 mg (31% yield) of4,4'-thiobis(N-phenylphthalimide) as a yellow solid. Trituration withdichloromethane afforded a white solid: mp 275°-280° C.; (lit 293°-295°C.--see U.S. Pat. No. 3,989,712); IR (KBr) 3065, 1710,1375 cm⁻¹ ; ¹ HNMR (slurry in DMSO-d₆) 7.52 (m,5H), 7.97 (m, 3H).

EXAMPLE 13

4,4'-Thiobis(Dimethyl Phthalate)

A mixture of 200 mg (0.47 mmol) of 4,4'-thiobis(dimethyl3,6-tetrahydrophthalate) and 213 mg (0.94 mmol) of2,3-dichloro-5,6-dicyano-l,4-benzoquinone (0.94 mmol) in 2 mL of toluenewas heated to reflux for 1 hour, allowed to cool to room temperature,and filtered to remove precipitated brown solids. The solids were washedwith 1 mL of toluene and the combined filtrates were concentrated invacuo. The residue was dissolved in 1 mL of dichloromethane and passedthrough a column of 1.0 g of neutral alumina (act. grade I). The columnwas washed with an additional 10 mL of dichloromethane. Combination andconcentration of the eluents afforded 208 mg of 4,4'-thiobis(dimethylphthalate) as a yellow oil (containing some toluene and dichloromethaneby NMR analysis, theoretical yield is 198 mg): ¹ H NMR (CDCL₃) 3.92 (s,6H), 7.47 (dd, 1H, J=8, 2 Hz), 7.66 (d, 1H, J=2 Hz), 7.74 (d, ¹ H, J=8Hz); ¹³ C NMR (CDCl₃) 52.5 (q), 52.6 (q), 129.9 (d), 130.3 (s), 130.4(d), 132.7 (d), 133.4 (s), 138.6 (s), 166.7 (s), 167.0 (s); IR (KBr)3000, 2951, 1724, 1586, 1556, 1433, 1292 1192, 1129, 1101, 1068, 969,772 cm⁻¹. Anal. Calcd. for C₂₀ H₁₈ O₈ S: C, 57.41: H, 4.34. Found: C,57.09; H, 4.56.

EXAMPLE 14 4,4'-Thiobis(Dimethyl Phthalate)

A mixture of 200 mg (0.47 mmol) of 4,4'-thiobis(dimethyl3,6-tetrahydrophthalate) and 31 mg (0.97 mmol) of elemental sulfur washeated to 215°-225° C. (sand bath) for 10 minutes and allowed to cool toroom temperature. The resulting black oil was purified by preparativeTLC on silica gel (eluent: 1% methanol in dichloromethane) to give 173mg (87% yield) of 4,4'thiobis(dimethylphthalate) as an orange oil.

EXAMPLE 15 4,4-Thiobis(N-phenylphthalimide)

A mixture of 100 mg (0.21 mmol) of4,4'thiobis(N-phenyl-1,2,3,6-tetrahydrophthalimide), 3 mg (0.03 mmol) ofI₂, 1.6 mL(0.03 mmol) of 96% H₂ SO₄, and 1 mL of dimethylsulfoxide washeated to 100° C. for 18 hours, allowd to cool to room temperature, andpoured into 10 mL of water. The resulting precipitate was removed byfiltration and washed with water and absolute ethanol. A TLC analysis ofthis material indicated the presence of1,4'-thiobis(N-phenylphthalimide).

EXAMPLE 16 4,4'-Thiobis(Phthalic Anhydride)

A mixture of 100 mg (0.30 mmol) 4,4'-thiobis(3,6-dihydrophthalicanhydride), 3 mg (0.03 mmol) I₂, 1.6 mL (0.03 mmol) of 96% H₂ SO₄, and 1mL of dimethylsulfoxide was heated to 90°-100° C. for 2 hours andallowed to cool to room temperature to give a black solution. This wasconcentrated in a stream of dry nitrogen. An NMR spectrum of the residueindicated the presence of 4,4'-thiobis(phthalic anhydride).

Example 17 illustrates that use of ordinary aromatization procedureswith some bis(alicyclic) thioethers such as4,4'-thiobis(1,2,3,6-tetrahydrophthalic anhydride) tends to result inthe formation of thioethers with diene ring systems.

EXAMPLE 17

4,4'-Thiobis(1,2-Dihydrophthalic Anhydride)

Warming of a slurry of 50 mg (0.15 mmol) of4,4'-thiobis(1,2,3,6-tetrahydrophthalic anhydride) and 1 mL oftetrahydrofuran produced a nearly homogeneous solution that was allowedto cool to room temperature. This was treated with 41 mg (0.30 mmol) ofN-bromoacetamide, stirred for 15 minutes, and treated dropwise with 42mL (0.30 mmol) of triethylamine. The resulting heterogeneous mixture wasstirred for a couple of minutes and diluted with 8 mL of water. Removalof the precipitated solid by filtration followed by drying in vacuo overphosphorus pentoxide afforded 19 mg (38% yield) of a solid: mp 218 223°C.; IR (KBr) 3431, 2923, 1842, 1791, 1691, 414, 1262, 1061, 905, 713cm⁻¹. The product was deemed to comprise4,4'-thiobis(l,2-dihydrophthalic anhydride).

As indicated above, it is presumed that a tetraene is formed as anintermediate when the dihydrothiophene dioxides are heated in thepresence of a suitable Diels-Alder dienophile. However attempts toisolate 2,2'-thiobisbutadiene (the intermediate from3,3'-thiobis(2,5-dihydrothiophene-1,1-dioxide) were unsuccessful. Itsprobable existance in solution was indicated by GC/MS (note Example 18below) but removal of the solvent by distillation resulted in depositionof a rubbery solid, even in the presence of a radical inhibitor.

Example 18 shows that DTS may be converted to 2,2'-thiobisbutadiene atrelatively mild temperatures.

EXAMPLE 18 2,2'-Thiobisbutadiene

A mixture of 0.5 g of DTS and 2 mL of toluene was heated at reflux for 2h. Examination of the solution by GC/MS revealed the presence of asingle peak identified 2,2'-thiobisbutadiene: mass spectrum (70 eV), m/e(relative intensity) 138 (M⁺,61), 137 (23), 123 (39), 105 (25), 97 (33),85 (22), 79 (23), 71 (80), 59 (29), 58 (30), 53 (100), 51 (41), 50 (23),45 (78), 39 (32).

This invention is susceptible to considerable variation in its practice,the descriptions and exemplifications given above being merelyillustrative thereof. Accordingly, it is not intended that thisinvention be unduly limited by the foregoing disclosure. Rather, what isintended to be covered is within the spirit and scope of the ensuingclaims.

What is claim is:
 1. 4,4'-thiobis(1,2,3,6-tetrahydrophthalimide) whereineach nitrogen atom is substituted by a hydrogen atom or a monovalenthydrocarbon group.
 2. 4,4'-thiobis(1,2,3,6-tetrahydrophthalimide). 3.4,4'-thiobis(N-phenyl-b 1,2,3,6-tetrahydrophthalimide).